(1) Field of the Invention
This invention relates to a current limiting type circuit breaker comprising first and second contact arms respectively carrying contacts operatable to open and close and electrical circuit and a trip mechanism responsive to an output from a means for detecting a fault such as a short-circuit or overload condition to open the contacts, and more particularly to such a device in which the contact arms are mounted for pivotal movement in the direction away from each other due to an electromagnetic repulsive force caused by an over current flowing therethrough.
(2) Description of the Prior Art
As is well known in the art, the current limiting type circuit breaker usually includes a trip mechanism which operates to open the contacts when the detecting means detects an excessively large current, and thereby opening an electrical circuit. Japanese Utility Model Publication No. 49-44446 discloses a construction in which the contact arms so provided as to become parallel to each other in the closed position is driven to open the contacts due to an electromagnetic repulsive force caused by the excessively large current flowing therethrough before operation of the trip mechanism.
According to the above-described circuit breaker, the electromagnetic repulsive force increases the speed at which the contact arm is driven so as to open the contacts. As a result, increase of an arc current can be restricted. Two types of the circuit breaker having the above-described construction are known: One is a type in which one of the contact arms is driven so as to open the contacts, and the other is a type in which both contact arms are driven when the contacts are opened. The latter type surpasses the former type in limiting current.
U.S. Pat. No. 3,815,059 discloses a circuit interrupter in which a magnetic substance is added to the latter of the above-described two types of circuit breaker so that the current limiting effect can be further improved. The magnetic substance is so placed as to provide paths for magnetic fluxes induced around the contact arms. Consequently, decrease of the magnetic resistance of the magnetic fluxes increases the electromagnetic repulsive force in magnitude.
Heightening the current limiting effect of the circuit breaker, that is, increasing the speed at which the contact arm is driven has recently desired. However, the prior art circuit breaker has a defect that the contact arm, when driven to the closed position, tends to rebound under the influence of the electromagnetic repulsive force as the driven speed thereof is increased, so that the contacts are reclosed.